The pro-Moscow leader of Ukraine's flashpoint Crimea peninsula called Friday on Russian-speaking eastern regions of Ukraine to hold their own referendums on switching over to Kremlin rule.

"If a sufficient number of people support it in those regions, then they should also have referendums," self-declared prime minister Sergiy Aksyonov told reporters ahead of Sunday's referendum in Crimea, which has been backed by Moscow but denounced as illegal by Kiev and much of the international community.

Ukraine moved Thursday to mobilize a volunteer force to ward off Russia's expansionist threat as Berlin warned Moscow of long-term damage to its economy and EU relations over the Crimea crisis.

The Verkhovna Rada parliament unanimously backed the creation of a new force of up to 60,000 volunteers who could keep Russian troops from advancing beyond the Crimean peninsula they seized at the start of the month.

The East-West split re-opened by the crisis in Ukraine hardened Wednesday when President Barack Obama threw Washington's weight firmly behind Ukraine in its stand-off with Moscow.

Obama welcomed Kiev's interim premier Arseniy Yatsenyuk to the White House and stood by his side as both leaders sternly warned Russia that Ukraine would not surrender its sovereignty.

Ukraine's former prime minister, Yulia Tymoshenko, has experienced some relief from debilitating back pain since arriving in Germany for care last week, the hospital treating her said Tuesday.

Tymoshenko, a leader of the pro-Western Orange Revolution in 2004, suffers from herniated discs and underwent medical examinations at Berlin's renowned Charite university hospital after landing on Saturday.

The three ex-Soviet Baltic states Lithuania, Latvia and Estonia can count on the security provided by their membership of NATO and the EU, Germany said Tuesday amid East-West tensions over the crisis in Ukraine.

"I am here to say that the Baltic states will not be left alone. This is a joint problem for NATO and the EU," German Foreign Minister Frank-Walter Steinmeier said in Tallinn, Estonia, ahead of talks in neighbouring Latvia and Lithuania.

Behind barbed wire fences at a top-security site in a German forest, workers in hazard suits will soon destroy remnants of Syrian chemical weapons of a type first tested here during World War I.

Far from the battlefields of the three-year-old Syrian war, this remote high-tech facility, which usually destroys munitions from two world wars, will help eliminate mustard gas stocks from the arsenal of President Bashar Assad.

German Chancellor Angela Merkel called on Russia on Wednesday not to do anything to destabilize the situation in Ukraine after pro-Moscow forces took de-facto control of Ukraine's majority Russian Crimean Peninsula.

"Russia must desist from any taunting which could lead to a destabilization of the situation," Merkel's spokesman Steffen Seibert told a government news briefing.

Germany's military will fly around 50 Ukrainians injured during unrest in central Kiev to the EU country for treatment, officials said Wednesday.

The move follows a request by the Ukrainian government to the German embassy in Kiev for help with the group, the defense ministry said in a statement.

EU foreign policy chief Catherine Ashton urged Russia on Saturday to refrain from deploying its armed forces in Ukraine and to abide by international law.

"I deplore today's decision by Russia on the use of armed forces in Ukraine," she said in a statement. "This is an unwarranted escalation of tensions.

British Foreign Secretary William Hague has urged his Russian counterpart to de-escalate the situation in Ukraine's restive Crimea peninsula, ahead of a trip to Kiev on Sunday.

"Have spoken to Foreign Minister (Sergei) Lavrov to call for de-escalation in Crimea and respect for sovereignty and independence of Ukraine," Hague said in a Twitter message on Saturday.